CAPILLARY SEALING IN SEDIMENTARY BASINS - A CLEAR FIELD EXAMPLE

Pore fluid overpressures as high as 2 MPa (full lithostatic) are obser ved at ODP Site 975 starting at depths of 145 mbsf (meters below the s ea floor). Sediment porosity increases to near surface values below th e top of overpressure, while compaction and pressures above the seal a re normal. Spikes in the density log and degassing of core from the ov erpressured zone indicate the presence of gas. The high porosity sedim ents are far too permeable to allow the generation and maintainance of fluid overpressures by disequilibrium compaction alone. The presence of two fluid phases in a layered sequence of fine and coarse sediment suggests capillary sealing, and this is shown to be quantitatively fea sible.